Population estimates and demographic profiles are central to both academic and public debates about immigration, immigrant assimilation, and minority mobility. Analysts’ conclusions are shaped by the choices that survey respondents make about how to identify themselves on surveys, but such choices change over time. Using linked responses to the 2000 and 2010 Censuses, our paper examines the extent to which individuals change between specific Hispanic categories such as Mexican origin. We first examine how changes in identification affect population change for national and regional origin groups. We then examine patterns of entry and exit to understand which groups more often switch between a non-Hispanic, another specific origin, or a general Hispanic identification. Finally, we profile who is most likely to change identification. Our findings affirm the fluidity of ethnic identification, especially between categories of Hispanic origin, which in turn carries important implications for population and compositional changes.